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Thread: Changing cam lift and duration.

  1. #31
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    Correct, I was not saying to keep your valves open as long as possible, I was saying maximise the fully open time for a given phase requirement (i.e. you want to open and close the valves to fully open/closed as quickly as possible and hold them there for as much of your open duration as you can). An ideal valve (as far as minimising pumping losses goes) would fully open instantaneously, hold open, and then close instantaneously (I think this is what F1 were trying to achieve with their pneumatic systems). I think this is true of any engine (a valve on its way up or down is simply adding friction (mechanical and aerodynamic) to the equation). However, with our restricted engines, a higher maximum lift cam won't necessarily work (as you have said), which is why some teams regrind for smaller lift (increasing the aerodynamic friction to create swirl, thus improving combustion efficiency even if the pumping efficiency side is reduced).

    Anyway, to work out the safe mm/degree of your theoretical cam, you need to follow the advice I gave above regarding valve bounce. Essentially you need to work out the mass of the valve assembly, how fast it is going to move due to a given cam profile at a given maximum RPM, how much the spring is going to resist, and whether or not the momentum caused in the valve assembly in this system is enough to overcome the force that the spring applies to keep the valve/follower in contact with the cam lobe. Given that you appear to be interested in reducing lift you probably won't have to worry about valve bounce. So I would take tromoly's advice and analyse the stock profile (using the experimental method I outlined above), which will produce a pretty picture. You can then essentially scale down that picture to create your cam profile. Just make sure you don't go crazy with ramp angles (i.e. KISS).
    Jay

    UoW FSAE '07-'09

  2. #32
    Thanks tromoly. Jay thanks for the info. Sorry I could not get it in the previous posts itself. I think you are rite about not worrying about valve bounce as I'm decreasing the duration. I will try to get the stock cams profile and than scale it down. Thanks guys.

  3. #33
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    I would advise you to change cam timings first, because this can be done easily and has a big effect.
    Changing the duration of the cams without any calculations of the influence it has on your gas exchange is pretty much useless.

    If you know what cam duration you want to have (simulated for example with a 1D engine Simulation like GT-Power or Ricardo Wave) , then you can follow Jays way to estimated the Forces in the Valvetrain.
    Besides the Valve, Retainer, and Lashdisk mass you will also have to calculate with the active spring mass which is typically around 1/3 to 1/2 of the springs weight.
    Another thing to factor is the natural frequency of the spring which is quite low and causes the spring to swing thus reducing the actual "usable" force of the spring (a big reason for F1 teams to go for pneumatic springs, because their natural frequency is much higher). So you should have at least a 30% gap between the spring force and the mass forces of your valve train to be on the save side.

  4. #34
    I am not getting it. How changing the cam timings without changing the duration can help. If I am not changing duration and only change the time at which intake and exhaust valve closes, how will it help in increasing the performance. Coming to the forces on the spring, if I am just decreasing the duration, the amount of forces acting on the springsis less than the stock valve trains forces. So why do I have to consider the forces acting on the valves. Please correct me if I am wrong.
    Rajeev Devaragudi
    Formula Manipal Season 2014-

  5. #35
    Quote Originally Posted by rajeevdevaragudi View Post
    I am not getting it. How changing the cam timings without changing the duration can help. If I am not changing duration and only change the time at which intake and exhaust valve closes, how will it help in increasing the performance. Coming to the forces on the spring, if I am just decreasing the duration, the amount of forces acting on the springsis less than the stock valve trains forces. So why do I have to consider the forces acting on the valves. Please correct me if I am wrong.
    The IVC angle is extremely critical to a lot of things (both performance and efficiency, which usually prefer different IVC angles). So, just changing the cam centerline timing, changes the IVC timing, which can change performance and efficiency quite a bit.

    Specifically, the IVC angle tells you what the effective compression ratio is (IVC at BDC produces highest CR), due to either LIVC or EIVC (late IVC or early IVC). Some engines use this to their advantage, drastically reducing CR with early or late IVC, allowing a higher mechanical CR for a higher ER (higher thermal efficiency) while reducing peak cylinder pressures and displacement (as a means of load control).

    IVO angle sets the overlap, which changes primarily the residual (internal EGR).

    For the exhaust valves, the EVO angle sets the blowdown volume. Less blowdown (later EVO) keeps mechanical expansion ratio (ER) high for thermal efficiency, but more blowdown helps use the in cylinder pressure to reduce exhaust pumping work, so it's always a balance. Turbocharger load can also change the optimal EVO angle.

    EVC angle again sets the overlap, which changes the residual like IVO angle.

    So the duration is relevant to the IVC and IVO angle, if you set IVC and IVO (or Overlap) targets then you can set your duration.

    In general, the only time I have specified more duration is for Late IVC cam phasing, with very long duration intake cams (to keep a bit of overlap).

    tl;dr, the IVC angle is what you really want to target
    Andrew Palardy
    Kettering University - Computer Engineering, FSAE, Clean Snowmobile Challenge
    Williams International - Commercial Turbofan Controls and Accessories

    "Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack

    "Any sufficiently advanced technology is indistinguishable from magic" ~Arthur C. Clarke

  6. #36
    Totally agree with you. But by changing the closing and opening timings of the IV I am effectively changing the duration itself. My main aim is to Chang the valve opening and closing times and also change the overlap as well. To decide these timings I am using Ricardo and I have still not decided the suitable timing. My main concern till day is how am I supposed to grind the cams to the required profile. If you have any ideas regarding this please feel free to post!!
    Rajeev Devaragudi
    Formula Manipal Season 2014-

  7. #37
    I would start by looking at the available aftermarket cams that fit your engine. For example, we run a WR450F engine, but can use cams from the YFZ450 quad engine as well, and have investigated most available aftermarket and stock cams from the WR and YFZ engines (we currently have a Hot Cams intake cam and stock quad exhaust cam).

    As long as the duration and lift are to the spec you want, you can phase the cam to get the IVO and IVC angles you want.

    What engine do you use?

    On a very unrelated note, I once had the pleasure of working on an engine that could vary the valve angles (through Multiair), and playing with the valve IVO and IVC angles in 'real time' and driving it around is probably the coolest thing I have done with an engine.
    Andrew Palardy
    Kettering University - Computer Engineering, FSAE, Clean Snowmobile Challenge
    Williams International - Commercial Turbofan Controls and Accessories

    "Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack

    "Any sufficiently advanced technology is indistinguishable from magic" ~Arthur C. Clarke

  8. #38
    I guess you have not read the previous posts of this thread. Aftermarket cams are not an option as they are more aggressive cams than the stock ones but the required cams for FSAE as we run our engines under restriction. Aftermarket cams are not preferable for engines under restriction. I have not actually searched for cams of other engines suitable for CBR 600RR ( the engine which we use BTW ). I don't really know how feasible that can be and it is really difficult to find such type of cams. Most of the camshaft mamfacturers whom I have spoke with have advised me to grind the stock cams to the required profile. So I need to grind the cams to a custom profile rather than going for aftermarket cams.
    Rajeev Devaragudi
    Formula Manipal Season 2014-

  9. #39
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    Okay i am trying to clear things up a bit.

    As apalrd said the IVC (intake valve closing) is the most important part of the cam timings and is very influential for cylinder filling for a naturally aspirated engine.
    A correct timing of the IVC allows you to use the inertia of the fresh gas to increase the cylinder filling:
    During the intake phase the gas in the intake runners is accelerated. Now when the Cylinder is moving up again the kinetic energy of the fresh gas can be converted into a charge effect which increases the pressure and density of the gas charge in the cylinder. With increased density and air mass your power is increased.
    In order to fully use this effect the IVC timing has to be appropriate. If the IVC is too early the charging effect is reduced because the pressure wave hits the valve while its closed. If the IVC is too late the piston moving up will press fresh air back into the intake runners, reducing the cylinder charge. For lower revs you will need an earlier IVC then for higher revs because the kinetic energy of the fresh gas is lower.

    Finding the right IVC (which will probably be earlier then for stock configuration because of the reduced revs in FSAE application) will yield the biggest gains that can be found with changed cam timings and durations.
    Determining the cam timings always starts at IVC.

    apalrd has already explained the exhaust valve timings very good so i will only add a bit to the valve overlap.
    As apalrd has said changing the valve overlap will have an effect on the residual gas mas in the cylinder. A higher overlap will generally reduce the residual gas mas in the cylinder but also increase the pumping losses. (with a proper set up of intake and exhaust runner lengths, the intake pressure during valve overlap is higher then the exhaust pressure, helping to flush the residual gas out of the cylinder) Another thing to keep in mind is, that with a high overlap you might loose fresh air charge going through the exhaust valve which is something that should be prevented especially for an engine with an air restrictor.

    I can only repeat myself: If you dont understand the effects of the basics its not useful to think of new cam durations. Try out different cam timings on the dyno, look at the data and try to understand whats happening. AFTERWARDS you can think of new valve lift profiles.
    Ricardo Wave can help you to understand the effects of the gas exchange. Look at the pressure at the valves and the mass flow through the valves during your gas exchange.


    Regarding the spring forces:
    Reducing the duration while maintaining the same valvelift will increase the accelerations (less time, same valve movement -> higher acceleration) and thus the forces. Looking at cam timings and valvetrain dynamics its always useful to look at the accelerations of a valve lift curve. Its actually the way they are designed. You are designing the acceleration curve and from there on you integrate to the actual valve lift curve. Its important to do it this way round, because you will want to have a smooth acceleration curve.

  10. #40
    I agree with RenM that cam timing and valve overlap are probably the most important things to get right.

    But on the other hand (and if you get the timing right), the CBR 600RR WILL in fact benefit from an intake cam with shorter duration.
    Now before you think aftermarket, you might take a closer look at your engine, and realize that there are two (2) camshafts in that engine. When you do some measurements (or spend about 5 minutes of searching the www), you will find out that one of these two camshafts has shorter duration and less lift than the other.
    Maybe you try to purchase another one of these instead of some aftermarket stuff...?
    Jan Dressler
    07 - 09 High Speed Karlsruhe / UAS Karlsruhe: Engine & Drivetrain Team
    09 - 10 High Speed Karlsruhe / UAS Karlsruhe: Engine & Drivetrain Team Leader
    10 - 13 High Speed Karlsruhe / UAS Karlsruhe: hanging around & annoying the team with random FSAE wisdom
    13 - ?? Gätmo Motorsport

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